The primary objective of the present investigation is to determine by experimental morphological techniques the ultrastructural localization of synaptic endings within the inferior olivary nucleus and the lateral reticular nucleus whose cell bodies of origin are in the red nucleus and spinal cord gray. This analysis is designed to determine the anatomical substrates for physiologically identified synaptic inputs and the anatomical basis for possible synaptic integration in two important nuclear sources of cerebellar afferents. A second goal is to evaluate the neurophysiological events within the inferior olive following rubral and spinal cord stimulation and thus determine the physiological action of the anatomically identified synaptic endings from each nuclear source. BIBLIOGRAPHIC REFERENCES: Mihailoff, G.A. and J.S. King, 1975. The basilar pontine gray of the opossum: A correlated light and electron microscope analysis. J. Comp. Neur., 159: 521-552. Martin, G.F., R. Dom, J.S. King, M. RoBards and C. Watson, 1975. The inferior olivary nucleus of the opossum, Didelphis marsupialis virginiana. Its organization and connections. J. Comp. Neur., 160: 507-534.